Radiator and apparatus including the radiator

ABSTRACT

A radiator  1  according to the present invention converts heat into electromagnetic waves and then radiates the electromagnetic waves through its surface. A number of microcavities are made in at least some areas on the surface, and the surface of the microcavities  2  is covered with a layer including tungsten that is bonded to carbon.

This is a continuation of International Application PCT/JP2005/001130, with an international filing date of Jan. 27, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radiator with a microcavity structure for improving its radiation efficiency in a particular wavelength range.

2. Description of the Related Art

An incandescent lamp, used extensively today as a common illumination source, includes a filament functioning as a thermal radiator. The “thermal radiator” is a radiation source that emits electromagnetic waves by thermal radiation. And the “thermal radiation” means radiation (of electromagnetic waves) produced by applying heat to atoms or molecules of an object. The thermal radiation energy is determined by the temperature of the object and has a continuous spectrum. In the following description, the thermal radiator will be simply referred to herein as a “radiator”.

An incandescent lamp achieves an excellent color rendering index and can be lit by a simple appliance. However, as the incandescent lamp uses a radiation produced by a filament that is generating heat, the radiation produced by the incandescent lamp in the visible wavelength range is just 10% of the overall radiations thereof (in a situation where the operating temperature is 2,600 K, for example). More specifically, the majority of the other radiations are infrared radiations, of which the energy density accounts for as much as about 70% of that of the overall radiations. Also, the current incandescent lamp causes a heat conduction due to an enclosed gas or a heat loss of as much as about 20% due to convection and have a visible radiation efficiency of only about 15 lm/W. Thus, various techniques of improving the visible radiation efficiency by cutting down the infrared radiations, which account for about 70% of the overall electromagnetic waves emitted from the radiator, have been researched.

U.S. Pat. No. 5,079,473 discloses a radiator including an array of micro-waveguides (which will be referred to herein as “microcavities”) on its surface. This radiator propagates only electromagnetic waves, of which the wavelengths are shorter than a predetermined wavelength that is defined by the shape and dimensions of the cavities, thereby cutting down the infrared radiations. According to U.S. Pat. No. 5,079,473, the cavities do not propagate electromagnetic waves if the wavelengths of the rays are twice or more as long as the inside diameter of the cavities. Thus, if the cavities have an inside diameter of 350 nm and the wall portions between the cavities have a thickness of 150 nm, then photons with wavelengths of 700 nm or more can be radiated only through those wall portions. However, no infrared electromagnetic waves with wavelengths of 700 nm or more will be propagated through the array of cavities.

When these design parameters are adopted, the sum of the areas of the array of cavities will account for 50% of the overall area of a plain surface with no cavities at all. According to U.S. Pat. No. 5,079,473, the total radiated rays with wavelengths exceeding 700 nm can be reduced to about one-tenth compared to tungsten at the same temperature, and the visible radiation efficiency can be approximately six times as high as the conventional one at an operating temperature of 2,100 K.

The spectrum of thermal radiation in thermal equilibrium state depends on the temperature following the Planck radiation formula. FIG. 1 is a graph showing the temperature dependence of blackbody radiation. In FIG. 1, the ordinate represents the spectral radiance B_(λ)Δλ [W·cm⁻² str⁻¹] (where Δλ10 nm) of blackbody, while the abscissa represents the wavelength [μm] of radiation. If the operating temperature of an incandescent lamp is 1,600 K, for example, then the spectral radiance distribution of the light radiated from its filament is represented by a curve with “1600 K” in this graph. This curve shows that the peak is located at a wavelength of about 2 μm and that infrared radiations account for a high percentage.

As is clear from FIG. 1, if the temperature of the radiator increases from 1,200 K to 2,000 K, then the radiation in the visible range increases by three orders of magnitude or more but the radiation in the infrared range does not change so much. That is why to obtain visible radiation efficiently, the operating temperature is preferably set to at least 2,000 K. Particularly when the radiator is used as an illumination source, the operating temperature should not be lower than 2,000 K because the resultant light would be excessively reddish if the operating temperature fell short of 2,000 K. For that reason, the radiator is preferably made of a refractory material such as tungsten that can withstand a high-temperature operation at 2,000 K or more.

The present inventors actually made such an array of cavities on the surface of a tungsten radiator and carried out various experiments on that radiator. As a result, in a tungsten radiator with an array of microcavities, each having a size of 1 μm or less, that array of cavities collapsed in a short time at a temperature of about 1,200 K, which is a sort of unusual and curious phenomenon. As described above, the filament of an incandescent lamp needs to operate at as high a temperature as 2,000 K or more and the incandescent lamp should have a sufficiently long life. If that specially designed cavity array structure, which has been reduced to a sub-micron dimension in order to minimize the infrared radiations, collapsed so easily, then such a radiator would no longer be applicable to an incandescent lamp and other devices that need to operate at elevated temperatures.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, an object of the present invention is to provide a radiator that can operate with good stability at such elevated temperatures even if the array of cavities arranged on the surface of the radiator has a very small size of 1 μm or less.

Another object of the present invention is to provide an incandescent lamp that includes such a radiator and that can emit visible radiation efficiently.

Yet another object of the present invention is to provide a non-illumination apparatus including the radiator and a method of making the radiator.

A radiator according to the present invention converts heat into electromagnetic waves and then radiates the electromagnetic waves through its surface. A plurality of microcavities are made in at least some areas of the surface, and the areas have a layer including tungsten and carbon.

In one preferred embodiment, the layer including tungsten and carbon contains tungsten that is bonded to carbon.

In another preferred embodiment, the microcavities make an array in at least those areas.

In another preferred embodiment, each of the microcavities is a recess with an inside diameter of 1 μm or less and a depth that is greater than the inside diameter.

In another preferred embodiment, the microcavities are arranged regularly at a pitch of 2 μm or less.

In another preferred embodiment, the microcavities are defined by gaps between a number of columnar members arranged.

In another preferred embodiment, the radiator has a body that is made essentially of tungsten.

In another preferred embodiment, the radiator is made essentially of tungsten carbide.

In another preferred embodiment, the radiator has an operating temperature of 2,000 K or more.

An apparatus according to the present invention includes a radiator according to any of the preferred embodiments of the present invention described above, a container for shutting off the radiator from the air, and energy supply means for supplying the radiator with energy and making the radiator emits electromagnetic waves.

A thermoelectric converter according to the present invention includes a radiator according to any of the preferred embodiments of the present invention described above, a container for shutting off the radiator from the air and a converter, which receives electromagnetic waves that has been radiated from the radiator and converts the electromagnetic waves into electric energy. The thermoelectric converter supplies the radiator with energy, thereby making the radiator radiate the electromagnetic wave.

A radiator making method according to the present invention is a method of making a radiator that converts heat into electromagnetic waves and then radiates the electromagnetic waves through its surface. The method includes the steps of: providing a tungsten member; making a plurality of microcavities in at least some areas on the surface of the tungsten member; and carbonizing at least some of the areas on the surface of the tungsten member.

Another radiator making method according to the present invention is a method of making a radiator that converts heat into electromagnetic waves and then radiates the electromagnetic waves through its surface. The method includes the steps of: providing a member that has a layer including tungsten and carbon in at least some areas on its surface; and making a plurality of microcavities in at least those areas on the surface of the member.

In one preferred embodiment, the layer including tungsten and carbon contains tungsten that is bonded to carbon.

In another preferred embodiment, the step of making a plurality of microcavities includes making the microcavities by laser irradiation or sandblasting.

Yet another radiator making method according to the present invention is a method of making a radiator that converts heat into electromagnetic waves and then radiates the electromagnetic waves through its surface. The method includes the steps of providing a number of wires, each having a layer that includes tungsten and carbon in at least some areas on its surface, and bundling the wires together, thereby making a plurality of microcavities in gaps between the wires.

In one preferred embodiment, the layer including tungsten and carbon contains tungsten that is bonded to carbon.

According to the present invention, carbon is introduced into some surface areas of tungsten, thereby increasing the thermal stability of the microcavity structure. As a result, a radiator with high radiation efficiency is realized by keeping the microstructure on the surface intact even at an elevated temperature and minimizing radiations having wavelengths that are equal to or greater than a predetermined wavelength. Also, an incandescent lamp according to the present invention, including such a radiator, realizes a luminaire that can convert thermal energy into visible radiation and then radiates the visible radiation with high efficiency.

In addition, since radiation efficiency can be increased in a particular wavelength range, the present invention can also achieve beneficial effects even when applied to various other types of apparatuses, not just illumination sources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the spectral radiances of blackbody radiation.

FIG. 2 illustrates a first specific preferred embodiment of a radiator according to the present invention.

FIGS. 3(a) through 3(e) are cross-sectional views schematically illustrating various relationships between a microcavity and a tungsten compound layer.

FIG. 4 is an SEM photograph showing the surface of tungsten that has been subjected to a carburizing process.

FIG. 5 is a graph showing the results of measurements by X-ray photoelectron spectroscopy (XPS).

FIGS. 6(a) and 6(b) are surface SEM photographs of a radiator as a comparative example that has not yet been heated and the same radiator that has been heated, respectively.

FIGS. 6(c) and 6(d) are surface SEM photographs of a radiator 1 as a preferred embodiment of the present invention that has not yet been heated and the same radiator that has been heated, respectively.

FIG. 7 is a graph showing the concentrations (partial pressures) of saturated oxygen that contributes to the oxidation reaction of tungsten.

FIG. 8 is a graph showing the Gibbs free energy of a refractory material during the oxidation reaction.

FIG. 9 is a graph showing the emissivity values of tungsten (W) and tungsten carbide (WC).

FIG. 10 schematically shows the microcavity collapse temperatures and melting points of tungsten and tungsten carbide.

FIG. 11 illustrates an exemplary configuration for an incandescent lamp including a radiator 1 according to a preferred embodiment of the present invention.

FIG. 12 illustrates an electrode that has been formed by going through a carburizing process.

FIG. 13 schematically illustrates a preferred embodiment of a thermoelectric converter according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of radiators according to the present invention will be described with reference to the accompanying drawings.

Embodiment 1

First, referring to FIG. 2, shown is a plan view schematically illustrating the surface of a radiator 1 according to a first specific preferred embodiment of the present invention. In FIG. 2, the dashed-line rectangle is a schematic representation showing a part of the surface of the radiator 1 on a larger scale.

The radiator 1 of this preferred embodiment has a ribbon shape as a whole with a width of 0.1 mm, a length of 10 mm and a thickness of 0.05 mm, and is made essentially of tungsten. On the surface of the radiator 1, provided is an array of cavities 2, each of which has a cylindrical shape with a diameter of 0.7 μm and a depth of 1.2 μm. These cavities 2 have a size of 1 μm or less each on a plane that is defined parallel to the radiation plane, and will be referred to herein as “microcavities”.

In this preferred embodiment, these microcavities 2 are arranged substantially regularly on the surface of the radiator 1 and the arrangement pitch (i.e., the distance between the center axes of two adjacent cavities) is set to 1.4 μm.

Those microcavities 2 can be made by any of various fine processesing technologies. In this preferred embodiment, the microcavities 2 are made by irradiating the surface of the radiator with a pulsed laser beam. Such a method of making very small recesses on the surface of a workpiece using a pulsed laser beam is described in Japanese Patent Application Laid-Open Publication No. 2001-314989, for example. More specifically, in this preferred embodiment, the fine processing is carried out by irradiating the radiator with a laser beam having a pulse energy of 0.1 mJ and a pulse width of 100 femtoseconds. The radiator 1 is repeatedly exposed to these laser pulses several tens to several thousands of times to make just one microcavity 2.

The radiator 1 to be processed with the laser beam is put on an X-Y stage. By irradiating the radiator with the laser beam synchronously with the movement of the X-Y stage, an array of microcavities such as that shown in FIG. 2 can be made. And if the movement of the X-Y stage is controlled with high precision, then the arrangement pattern of the array can be defined arbitrarily. In this preferred embodiment, the microcavities 2 are arranged regularly at a substantially constant pitch. Alternatively, the density of the microcavities 2 may be set high in one area but low in another such that the radiator 1 has radiation properties that change from one position to another. The inside diameter and depth of the microcavities 2 can be set to arbitrary values by adjusting the irradiation energy density, beam spot size, number of shots and other parameters of the laser pulses.

Optionally, to make a huge number of microcavities at the same time, the photolithography and etching techniques, which are used extensively in semiconductor device fabrication and micro-electro-mechanical systems or MEMS technologies, may also be adopted.

The prime feature of the radiator 1 of this preferred embodiment is that a surface region of the radiator 1 to a depth of about 2 μm as measured from the radiation plane of the radiator 1 is a layer including tungsten and carbon. As will be described more fully later, at least a part of tungsten is chemically bonded to another element (such as carbon) in that layer including tungsten and carbon. Thus, this layer will be referred to herein as a “tungsten compound layer”.

In this preferred embodiment, the surface of tungsten is subjected to a carburizing process to make such a tungsten compound layer. The carburizing process is a process for carbonizing the surface of a metal, for example, and may be carried out by any of various methods that have been developed so far. For example, according to a plasma carburizing process, using the furnace body or thermal insulator as an anode and the workpiece as a cathode, respectively, a high DC voltage is applied between these electrodes in a rare gas atmosphere containing a hydrocarbon gas such as methane or propane including argon and hydrogen, thereby generating glow discharge and eventually plasma. In the plasma, various electrochemical reactions set in to make ions of the hydrocarbon gas bombard on the surface of the workpiece and thereby produce carburizing. This plasma carburizing process is more effective in activating, cleaning or reducing the surface of the workpiece than any other type of carburizing process. In a preferred embodiment, the carburizing process is preferably carried out at a temperature of 500° C. to 2,000° C. (e.g., at 1,100° C.) for 4 to 48 hours (e.g., for 8 hours). By modifying the carburizing process conditions, the thickness of the resultant tungsten compound layer can be controlled. To improve the thermal stability, it should be enough to deposit the tungsten compound layer to a thickness of at least about several nanometers.

However, the tungsten compound layer does not have to be made by such a carburizing process but may also be formed by introducing a constituent element of the compound, such as carbon, into tungsten by either ion implantation or solid-phase diffusion.

In this preferred embodiment, an array of microcavities 2 is made on the surface of tungsten and then the workpiece is subjected to the carburizing process. That is why the workpiece has a surface area that is big enough to carbonize the tungsten efficiently. Alternatively, tungsten may be subjected to the carburizing process first, and then processed to make the array of microcavities 2 thereon. In that case, a compound layer may be formed to a thickness that falls short of the depth of the microcavities 2 to make. This is because the array structure of the microcavities 2 can be thermally stabilized just by forming such a thin compound layer on the surface.

FIGS. 3(a) through 3(e) are cross-sectional views schematically illustrating various relationships between the microcavity 2 and the tungsten compound layer 22. In FIG. 3(a), a tungsten compound layer 22, of which the thickness is smaller than the depth of the microcavity, has been formed on the surface of tungsten 21. In the example illustrated in FIG. 3(b), the tungsten compound layer 22 is even thinner than the counterpart shown in FIG. 3(a). FIG. 3(c) illustrates a situation where a tungsten compound layer 22 has been formed on the surface of tungsten 21 and then a microcavity is made there. In this configuration, there is no tungsten compound layer 22 on the bottom and side surfaces of the microcavity 2. Even so, the microcavity structure is also thermally stabilized. The reasons are as follows. In the prior art, a microcavity structure that has been defined on the surface of tungsten easily collapses at a relatively low temperature as described above. This may be because the tungsten atoms migrate too actively while tungsten is being heated by electrical currents. To reduce such migration of atoms, the entire surface of the microcavity structure is preferably covered with a compound layer that minimizes the migration. However, only the edge portions, where the structure loses its stability most easily, may be coated with that tungsten compound layer.

FIG. 3(d) illustrates a structure in which only the side surfaces of a microcavity 2 are coated with a tungsten compound layer 22. Such a structure can be obtained by subjecting the structure shown in FIG. 3(b) to a physical etching process such that a portion of the tungsten compound layer 22, parallel to the principal surface, is removed thinly. This tungsten compound layer 22 is also present at the edge portions 23 of the microcavity. Accordingly, even this compound layer, covering just a small area as a whole, can contribute sufficiently effectively to stabilizing the structure of the microcavity.

FIG. 3(e) illustrates an example in which a broad area, including the microcavity, is entirely covered with the tungsten compound 22. A structure like this can be obtained by either subjecting the surface of tungsten to a carburizing process for a long time or using a tungsten compound such as tungsten carbide, obtained by a sintering process, for example, as the material of the radiator 1 as it is. In the latter case, a tungsten carbide member that has been cut to an appropriate size and shape is prepared and then patterned to make an array of microcavities thereon.

FIG. 4 is a scanning electron microscope (SEM) photograph showing a cross-sectional structure near the surface of tungsten that has been subjected to the carburizing process mentioned above. In FIG. 4, to clarify the cross-sectional layered structure, a carbon layer (C-deposition) is deposited on the surface of the sample with a Pt—Pd layer sandwiched between them.

As can be easily seen from FIG. 4, the layer produced by the carburizing process does not have as definite a polycrystalline structure as tungsten and seems to consist of amorphous phases or microcrystalline phases. In the sample shown in FIG. 4, the layer produced by the carburizing process has a thickness of about 1.8 μm.

FIG. 5 is a graph showing the results of measurements by X-ray photoelectron spectroscopy (XPS). In FIG. 5, the ordinate represents the intensity (or the count) of photoelectrons (i.e., 4f electrons of tungsten) that were emitted from the surface of a sample when exposed to an X-ray in a vacuum, while the abscissa represents the binding energy. The measurements were done by using an analyzer ESCA5400HC produced by Physical Electronics USA. Monochromated-A1Kα (14 KeV at 200 W) was used as an X-ray anode and the analysis area was a circle with a diameter of 0.6 mm.

As can be seen from these results of measurements, in the layer formed by the carburizing process (which will be referred to herein as a “carburized layer”), the 4f electron binding energy of tungsten made a chemical shift from the value of tungsten by itself in its crystals. Based on the results of other measurements, the present inventors confirmed that carbon in the carburized layer had a higher concentration than carbon in tungsten. Taking all of these results into account, at least a portion of tungsten in the carburized layer would be chemically bonded to another element (i.e., carbon) to make a compound.

Based on these results of measurements, the layer formed on the surface of tungsten by the carburizing process is referred to herein as a “tungsten compound layer”. However, the “tungsten compound layer” does not necessarily mean that all tungsten atoms included in that layer are bonded to carbon to make the compound layer. Rather the “tungsten compound layer” may refer to any layer in which carbon and at least a portion of a layer containing tungsten are chemically bonded together.

A radiator 1 according to this preferred embodiment and a radiator representing a comparative example, of which the surface had not been subjected to the carburizing process, were prepared and heated at 2,000 K for 10 minutes in a vacuum with a pressure of about 10⁻⁶ Torr. FIGS. 6(a) and 6(b) are surface SEM photographs of the radiator as a comparative example that had not been heated yet and the same radiator that had been heated, respectively. FIGS. 6(c) and 6(d) are surface SEM photographs of a radiator 1 as a preferred embodiment of the present invention that had not been heated yet and the same radiator that had been heated, respectively. The surface of the radiator 1 shown in FIGS. 6(c) and 6(d) is the tungsten compound layer described above.

As can be seen from FIGS. 6(a) through 6(d), the microcavity structure of the radiator 1 of this preferred embodiment did not change at all even after having been subjected to the heating test, while the microcavity structure of the radiator as the comparative example collapsed completely after the test with no traces left.

The evaporation rate of tungsten depends on the pressure of the atmospheric gas. The higher the degree of vacuum, the more easily tungsten evaporates. When the radiator 1 of this preferred embodiment is actually used as the filament of an incandescent lamp, the filament may be placed in an inert gas atmosphere with a pressure of 1 atm, for example. The life expectancy of the filament in such a situation may be estimated by a diffusion equation. According to the calculations, if the array of microcavities could be held stabilized at 2,000 K for 10 minutes within a vacuum of about 10⁻⁶ Torr, then the same array of microcavities would be kept stabilized at 2,000 K for approximately 9,700 hours when put in an inert gas atmosphere of 1 atm. Consequently, an incandescent lamp including the radiator 1 of this preferred embodiment is expected to have a life of approximately 10,000 hours, which is ten times as long as the life of 1,000 hours of a lamp that uses the conventional tungsten filament.

The present inventors discovered that when formed on the surface of a tungsten filament as in the comparative example, the microcavity structure collapsed and disappeared even at a low temperature of about 1,200 K. As described in “Metal Data Book” (Revised 3^(rd) Edition, edited by the Japan Institute of Metals and published by Maruzen Co., Ltd.), for example, tungsten has a melting point of 3653.15 K. Accordingly, it is unthinkable that the cavity structure of tungsten melted at as low a temperature as about 1,200 K.

Thus, the present inventors explored the possibility that the microcavity array structure on the surface of tungsten might have been thinly oxidized to decrease the melting point of the surface layer significantly.

FIG. 7 is a graph showing the partial pressures of saturated oxygen that contributes to the oxidation reaction of tungsten. In FIG. 7, the ordinate represents the partial pressure, while the abscissa represents the temperature. WO (s) and WO₂ (S), for example, represent the temperature dependence of the partial pressure of tungsten oxides in solid state. Meanwhile, WO (g) and WO₂ (g), for example, represent the temperature dependence of the partial pressure of tungsten oxides in gas state.

As can be seen from FIG. 7, at a low temperature around room temperature, the oxidation reaction of tungsten advances with a very small amount of oxygen in every tungsten oxide. Accordingly, even if hydrogen reduction or any other treatment is carried out to remove oxygen from the surface of tungsten, the surface will get oxidized easily as soon as exposed to the air again.

Likewise, a conventional tungsten filament, used in common incandescent lamps, would also be coated with a thin oxide layer when exposed to the air at room temperature during the manufacturing process thereof. However, as soon as the lamp is lit, such an oxide layer will vaporize away and the underlying tungsten will surface instead. That is why the characteristic of the incandescent lamps would not be affected by such an oxide layer.

On the other hand, if the array of microcavities is provided on the surface as in the present invention, it's a quite different story. Specifically, if the array of microcavities on the surface of tungsten is exposed to the air at room temperature and oxidized, then the microcavity structure itself will collapse and disappear when the lamp is lit.

FIG. 8 is a graph showing the Gibbs free energy of a refractory material during the oxidation reaction. Specifically, FIG. 8 shows the oxidation resistance.

As is clear from the chemical formulae shown in FIG. 8, to turn tungsten carbide into tungsten oxide, first, tungsten carbide needs to be oxidized to decompose the tungsten carbide into tungsten and CO and then the tungsten needs to be oxidized. And the reactions in these two stages need to occur continuously. Also, the reaction of decomposing tungsten carbide into tungsten and CO through oxidation does not occur so easily as the reaction of producing tungsten oxide from tungsten as shown in FIG. 8.

Thus, it can be seen that tungsten carbide is less easily oxidizable than molybdenum, niobium or tungsten. Tantalum carbide should also have a similar property.

Considering that tungsten carbide is less easily oxidizable than tungsten, the tungsten compound layer formed by the carburizing process of the present invention is also less easily oxidizable than tungsten and that property may thermally stabilize the microcavity structure.

Nevertheless, the compound layer of the present invention does not have to have the same property as tungsten carbide in bulk. The analysis results mentioned above show that tungsten is chemically bonded to another element in the layer formed by the carburizing process of the present invention on the surface of tungsten and that carbon is present in that layer at a higher concentration than in tungsten. Taking these results into consideration, it is clear that tungsten and carbon are chemically bonded together. However, since the present inventors could not confirm the exact composition, we cannot say this compound layer must be “tungsten carbide”. Nevertheless, we can at least say that this compound layer has a similar property to that of tungsten carbide. That is why the chances that tungsten carbide was produced at least partially are high. Consequently, the tungsten compound mentioned above is typically tungsten carbide but is not limited to this particular compound.

Although less easily oxidizable as described above, tungsten carbide still has not been used as the material of a filament for an incandescent lamp. This is partly because the melting point of tungsten carbide is lower than that of tungsten by as much as several hundreds K and partly because there is a big difference in emissivity between tungsten and tungsten carbide.

Hereinafter, the difference in emissivity will be described with reference to FIG. 9.

FIG. 9 is a graph showing the emissivity values of tungsten and tungsten carbide in the infrared range. As can be seen from FIG. 9, the emissivity of tungsten carbide (WC) in the infrared range is higher than that of tungsten (W) in the same range. For example, at a wavelength of 2.5 μm, tungsten has an emissivity of 20% while tungsten carbide has an emissivity of 70%. As a result, visible radiation accounts for a lower percentage of the overall radiations from tungsten carbide. That is why if a filament is made of tungsten carbide, the luminous efficacy in the visible radiation range would decrease so significantly compared with a tungsten filament that the tungsten carbide filament cannot be used to make a light bulb.

A history of development of incandescent lamps teaches us that light bulbs with a carbon filament having a low melting point and a high infrared emissivity (which are called “Edison bulbs”) were initially used for some time after the incandescent lamp was invented. After that, the carbon filament was gradually replaced with a tungsten filament having a higher melting point. Such a historical background formed a basis for a common technical misconception that tungsten carbide, having a lower melting point and a higher infrared emissivity than tungsten, should not be used for a radiator such as a filament.

On the other hand, the radiator of the present invention dares to use tungsten carbide, which has a relatively low radiation efficiency in the visible radiation range, but has a microcavity structure on its surface. That is why the radiator of the present invention can cut down infrared radiations sufficiently and can reduce the high infrared emissivity, which should be shown by tungsten carbide otherwise, to a rather low level.

In addition, since the radiator of the present invention can improve the radiation efficiency, the operating temperature can also be decreased compared with a situation where a tungsten filament is used.

Considering that the melting point of tungsten carbide is far lower than that of tungsten, it would be hard to imagine for those skilled in the art that the collapse of microcavities at an elevated temperature can be minimized by subjecting tungsten to the carburizing process.

FIG. 10 schematically shows the microcavity collapse temperatures and melting points of tungsten and tungsten carbide. As can be seen from FIG. 10, tungsten carbide (WC) has a melting point of about 3,175 K, which is lower than that of tungsten of about 3,650 K (according to “Metal Data Book”, Revised 3^(rd) Edition, edited by the Japan Institute of Metals and published by Maruzen Co., Ltd.) Nevertheless, the microcavity structure subjected to the carburizing process collapses at about 2,400 K. This temperature is much higher than the temperature (of about 1,900 K) at which the microcavities of tungsten collapse and is too high to predict from the melting point of tungsten carbide.

Hereinafter, a preferred embodiment of an illumination unit including a radiator according to a preferred embodiment of the present invention will be described with reference to FIG. 11, which illustrates an exemplary configuration for an incandescent lamp including the radiator 1.

This incandescent lamp includes a radiator (filament) 1 that emits light, a translucent bulb 12 for shutting off the radiator 1 from the air, a stem 13 for supporting an electrode that is connected to the radiator 1, and a base 14, which is electrically connected to the radiator 1 by way of the electrode to supply electric power to the radiator 1 from an AC outlet. An argon gas, for example, is preferably enclosed in the bulb 12 to minimize the evaporation of the filament.

In the incandescent lamp illustrated in FIG. 11, the radiator 1 has the thermally stabilized microcavity structure described above. Thus, even when operated at a temperature of 2,000 K, the lamp can keep emitting radiations, having a spectral distribution with small infrared radiations, for a long period of time.

A tungsten electrode, of which the surface is partially covered with a layer that has been formed by a carburizing process (i.e., a carburized layer), has been known in the art (see Japanese Patent Application Laid-Open Publications Nos. 9-111387 and 9-111388, for example). FIG. 12 schematically illustrates an exemplary configuration for such an electrode. The tungsten electrode 30 shown in FIG. 12 has a shape defined by sharpening one end of a circular tungsten rod with a length of 0.2 m to 15 m into a conical shape and then cutting off the sharpened end by 0.2 mm to 0.8 mm. This tungsten electrode 30 includes thorium and emits electrons through the sharpened end. The entire conical portion of the tungsten electrode 30, except the sharpened portion through which electrons are emitted, has been subjected to the carburizing process. The carburizing process is performed to prevent thorium atoms from diffusing through the crystal grain boundary of the tungsten electrode and going out of the electrode. In this case, the carburized layer formed by the carburizing process would be made of W2C, of which the melting point is lower than that of tungsten, and would be located in a region of the tungsten electrode 30 with a relatively low temperature (i.e., not the high-temperature end portion through which electrons are emitted).

Thus, it has been a technique that is well known in the art to subject a portion of a tungsten electrode for emitting electrons or thermo electrons to a carburizing process. However, nobody has ever reported subjecting a member to be used at an elevated temperature (such as a filament) to the carburizing process.

Optionally, to cut down radiations of which the wavelengths exceed a predetermined value, the surface of the radiator may be treated to have fine unevenness, defined by a huge number of recesses, by any method other than that described above such that each of those small recesses (with an average size of 1 μm or less) functions as a microcavity. For example, by treating the surface by sandblasting, a huge number of recesses functioning as microcavities can be made on the surface of the radiator. Even so, according to the present invention, the decrease in thermal stability due to oxidation of the surface of the radiator can be minimized and heat radiation can be emitted for a long time at an elevated temperature. Thus, the present invention is effectively applicable for use in a filament for an incandescent lamp, for example.

As described above, according to the present invention, the microstructure on the surface can be kept stabilized even at an elevated temperature of 2,000 K or more. However, such effects are achieved not only when recesses are made on the surface of a radiator but also when a more complicated microstructure is formed by a MEMS or any other fine-line patterning process. For example, a photonic crystal structure may be formed on the radiation plane of a radiator by arranging and stacking very small grid members and defining a lattice structure at an interval that is approximately equal to the wavelength of light. According to the present invention, the surface portion or all of the member that defines such a microstructure is made of tungsten carbide. Consequently, a microstructure that can improve the radiation efficiency in a selected wavelength range can be operated for a long time even at an elevated temperature.

Optionally, the radiator of the present invention is also applicable for use in a three-dimensional tungsten structure as disclosed in Pamphlet of PCT International Application Publication No. WO 03/058676A2. That is to say, the present invention can cope with the “collapse of a microstructure”, which is a big problem that could happen more and more often when a member that has been made of a material with a very high melting point such as tungsten to increase the thermal resistance is further downsized in the near future.

Embodiment 2

Hereinafter, a preferred embodiment of a thermoelectric converter will be described as a non-illumination apparatus that uses the radiator of the present invention.

FIG. 13 schematically illustrates a configuration for such a thermoelectric converter. The apparatus illustrated in FIG. 13 includes a radiator 40 according to a preferred embodiment of the present invention for absorbing sunray (as electromagnetic waves) and emitting electromagnetic waves with a particular wavelength, a container (not shown) for shutting off this radiator 40 from the air, and a converter (such as a photovoltaic cell) 44 that receives the electromagnetic waves from the radiator 40 and converts them into electrical energy. In the example shown in FIG. 13, a filter 42 for filtering out components with unnecessary wavelengths is arranged as an optional member between the radiator 40 and the converter 44.

The radiator 40 includes a body portion, which is made essentially of tungsten and of which the surface has a microstructure such as microcavities or a photonic crystal structure. The surface regions of the radiator 40, on which the microstructure (such as microcavities) for improving the radiation efficiency at the particular wavelength is provided, are covered with a layer including tungsten and carbon as in the first preferred embodiment described above. In this manner, the microstructure on the surface of the radiator 40 selectively emits electromagnetic waves with a particular wavelength, which should agree with the wavelength at which the converter 44 can absorb the electromagnetic waves efficiently.

If the radiator 40 is supplied with energy by exposing the radiator 40 to solar heat collected, for example, then the radiator 40 that has been heated to an elevated temperature (of 2,000 K or more) emits electromagnetic waves in a particular wavelength range. On receiving such electromagnetic waves by way of the filter 42, the converter 44 can convert the radiation into electrical energy highly efficiently.

A sunray usually includes a lot of electromagnetic waves, which fall within wavelength ranges that would result in low conversion efficiency by the converter 44. However, by using the radiator 40 of the present invention (and the filter 42), electromagnetic waves, falling within wavelength ranges that would result in high conversion efficiency, can be supplied to the converter 44. As a result, the overall conversion efficiency of an opto-thermo-electric energy conversion system can be increased. Such a thermoelectric converter can also generate electrical energy even by heating the radiator 40 with non-optical energy, and therefore, can be used in a generator for a non-opto-thermo-electric conversion system.

A thermal electromotive force generator system that uses a radiator with such wavelength selectivity is also disclosed in Japanese Patent Publication No. 347283, for example. However, this publication discloses only a radiator made of a tungsten material and does not mention at all that a microstructure would collapse due to heat.

In the preferred embodiment of the present invention described above, the thermal stability of the microcavity or photonic crystal structure on the surface of the radiator 40 is increased by the layer including tungsten and carbon. Thus, the reliability of the generator system can be kept high for a long time and the radiator 40 can be operated at even higher temperatures. As a result, even a significant increase in the output of generator systems can also be coped with flexibly. Consequently, the apparatus of this preferred embodiment can contribute to protecting the global environment as a generator system that uses sunrays.

While the present invention has been described with respect to preferred embodiments thereof, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.

This application is based on Japanese Patent Application No. 2004-075784 filed Mar. 17, 2004, the entire contents of which are hereby incorporated by reference. 

1. A radiator for converting heat into electromagnetic waves and then radiating the electromagnetic waves through its surface, wherein a plurality of microcavities are made in at least some areas of the surface, and wherein the areas have a layer including tungsten and carbon.
 2. The radiator of claim 1, wherein the layer including tungsten and carbon contains tungsten that is bonded to carbon.
 3. The radiator of claim 1, wherein the microcavities make an array in at least those areas.
 4. The radiator of claim 1, wherein each of the microcavities is a recess with an inside diameter of 1 μm or less and a depth that is greater than the inside diameter.
 5. The radiator of claim 1, wherein the microcavities are arranged regularly at a pitch of 2 μm or less.
 6. The radiator of claim 1, wherein the microcavities are defined by gaps between a number of columnar members arranged.
 7. The radiator of claim 1, wherein the radiator has a body that is made essentially of tungsten.
 8. The radiator of claim 1, wherein the radiator is made essentially of tungsten carbide.
 9. The radiator of claim 1, wherein the radiator operates at a temperature of 2,000 K or more.
 10. An apparatus comprising: the radiator of claim 1; a container for shutting off the radiator from the air; and energy supply means for supplying the radiator with energy and making the radiator emits electromagnetic waves.
 11. A thermoelectric converter comprising: the radiator of claim 1; a container for shutting off the radiator from the air; and a converter, which receives the electromagnetic waves that has been emitted from the radiator and converts the electromagnetic waves into electric energy, wherein the thermoelectric converter supplies the radiator with energy, thereby making the radiator radiate the electromagnetic waves.
 12. A method of making a radiator that converts heat into electromagnetic waves and then radiates the electromagnetic waves through its surface, the method comprising the steps of: providing a tungsten member; making a plurality of microcavities in at least some areas on the surface of the tungsten member; and carbonizing at least some of the areas on the surface of the tungsten member.
 13. A method of making a radiator that converts heat into electromagnetic waves and then radiates the electromagnetic waves through its surface, the method comprising the steps of: providing a member that has a layer including tungsten and carbon in at least some areas on its surface; and making a plurality of microcavities in at least those areas on the surface of the member.
 14. The method of claim 13, wherein the layer including tungsten and carbon contains tungsten that is bonded to carbon.
 15. The method of claim 12, wherein the step of making a plurality of microcavities includes making the microcavities by laser irradiation or sandblasting.
 16. A method of making a radiator that converts heat into electromagnetic waves and then radiates the electromagnetic waves through its surface, the method comprising the steps of: providing a number of wires, each having a layer that includes tungsten and carbon in at least some areas on its surface; and bundling the wires together, thereby making a plurality of microcavities in gaps between the wires.
 17. The method of claim 16, wherein the layer including tungsten and carbon contains tungsten that is bonded to carbon. 